1. Field of the Invention
The present invention relates to a method of heating slabs by a side-burner type heating furnace, and particularly to a method of heating slabs by a heating furnace provided at its side walls with burners capable of controlling the flame length.
2. Description of the Prior Art
In general, slabs produced by a slabbing or a continuous casting are heated in a heating furnace up to a predetermined temperature necessary for rolling the slab. The heating furnace is generally used in the form of a continuous heating furnace from the view point of the efficiency, but may be used in the form of a batch system heating furnace in the heating of special steel. Particularly, in the heating of special steel, a so-called side-burner type heating furnace, that is, a box type furnace, which has no nose portion and is provided with burners at its both side walls, is generally used in order to heat uniformly a plurality of slabs in one batch and to utilize effectively the space of the furnace. However, in the conventional side-burner type heating furnace, temperature difference occurs in its width direction to cause difficulties in the uniform heating of a slab and to cause temperature difference in the length direction of the slab. As the result, the dimensional accuracy in the rolling of the slab is often adversely affected in both the continuous heating and the batch system heating. Such temperature difference in the width direction of the furnace is mainly due to the burning state of burners, and the temperature of a furnace is often higher is the center portion than in the side portion.
Further, the amount of heat to be supplied to a heating furnace for slab varies noticeably depending upon the variation of heating load, and is sometimes smaller than the lower limit of the amount of heat capable of maintaining the stable burning state of burners. In this case, when a conventional heating furnace is used, a high oxygen operation must be carried out in a high air/fuel ratio to maintain steadily the burning state of the burner. However, the high oxygen operation has such drawbacks that the burning temperature is low, a large amount of fuel is required, and further the concentration of nitrogen oxide in the waste gas is high.